The present application relates to a wireless communication device, a wireless communication method, and a computer program that performs a receive process of an ultra-wideband (“UWB,” hereafter) signal using a very wide frequency band. More specifically, the invention relates to a wireless communication device, a wireless communication method, and a computer program that respectively perform demodulation of a received multi-level modulated signal through a reduced number of calculations.
More specifically, the invention relates to a wireless communication device, a wireless communication method, and a computer program that perform the demodulation of a received multi-level modulated symbol by performing likelihood determination with respect to a reference signal point in a signal constellation through a reduced number of calculations. Yet more specifically, the invention relates to a wireless communication device, a wireless communication method, and a computer program that respectively simplify calculations for calculating the likelihood of a received QAM mapped symbol with respect to a respective reference symbol in a signal constellation.
FIG. 3 shows example frequency mapping specified relative to multiband OFDM UWB (“MB-OFDM,” hereafter) communication systems (see, “MBOFDM PHY Specification Final Release 1.0,” Wimedia Alliance, Apr. 27, 2005). In the example shown therein, a 5-GHz zone to be used by a wireless LAN is set to a null band, and the other zone is divided into 13 subbands. The subbands are divided into four groups A to D, whereby communication is performed by managing the frequency in units of the group.
FIG. 4 shows a state of data transmission being performed in MB-OFDM through frequency hopping with respect to OFDM symbols on a time axis. More specifically, in the shown example, a group A of bands #1 to #3 is used, and OFDM modulation using an IFFT (inverse fast Fourier transform)/FFT (fast Fourier transform) composed of 128 points is carried out via frequency hopping being performed while the central frequency is varied in units of one OFDM symbol.
FIG. 5 is a diagram showing 128 subcarriers in an OFDM symbol. In the diagram, 128 subcarriers in one OFDM symbol are shown, and the subcarriers correspond to one subband in FIG. 3 and one symbol frequency-hopped in FIG. 4. As shown in FIG. 3, in MB-OFDM, of 128 pieces of subcarriers, 100 pieces are used as data subcarriers for carrying transmission data, 12 pieces are used as pilot subcarriers for carrying well-known pilot signals, and six pieces are used as carrier holes (that is, subcarriers not having energy). Five pieces each of subcarriers located internally of carrier holes on both sides are used as dummy subcarriers that do not carry information, and five pieces are generated by copying from the end portion of the data subcarrier.
Table 1 below summarizes values used in the MB-OFDM, such as, for example, transmission rates, modulation schemes allocated for the respective transmission rates, and coding rates.
TABLE 1TransmissionModulationCoding RateConjugate SymmetryTime SpreadingSegment SpreadRate [Mbps]Scheme[R]Input for IFFTFactor [TSF]GainNCBPS39.4QPSK 17/69Yes2410053.3QPSK⅓Yes2410080QPSK½Yes24100106.7QPSK⅓No22200160QPSK½No22200200QPSK⅝No22200320DCM½No11200400DCM⅝No11200480DCM¾No11200* NCBPS: Coded bits per OFDM symbol
As shown in Table 1, the following modulation schemes are used in MB-OFDM. The QPSK (quadrature phase keying) modulation scheme is used for the transmission rates in the range of from 39.4 Mbps to 200 Mbps, and the DCM (dual-carrier modulation) scheme configured by combining frequency diversity scheme with 16 QAM (16 quadrature amplified modulation) scheme is used for the transmission rates in the range of from 320 Mbps to 480 Mbps. In the table, 80 Mbps and 160 Mbps are optional transmission rates.
QPSK and 16 QAM are general modulation schemes in respect of the multi-level modulation scheme that maps a multibit binary signal into a specific point in a signal constellation. For example, in the QPSK, a 2-bit binary signal is mapped into four signal points (transmission symbols) each having a different phase in the signal constellation corresponding to the combination of the 2-bit value of the binary signal. In 16 QAM, a 4-bit binary signal is mapped into 16 signal points each created by using a combination of phase and amplitude in the signal constellation corresponding to the combination of the 4-bit value of the binary signal.
In the DCM scheme, in the event of 16 QAM is used as the modulation scheme, of a total of 100 data subcarriers, data is superimposed onto 50 data subcarriers, and the same data is redundantly superimposed onto the remaining 50 data subcarriers. That is, the same information is divided for two carriers and transmitted thereover. In this case, both streams of data can be synthetically received and reproduced on the receiver side, therefore making it possible to obtain the effects of frequency diversity. In Table 1, while the coded bits per OFDM symbol are shown, 200 information bits are arrayed in one OFDM symbol for the rate higher or equal to 320 Mbps. In the 16 QAM scheme, four bits can be transmitted with one symbol. A 16-QAM symbol is generated as shown in Expressions (1) to (3) below.
                              d          ⁡                      (            k            )                          =                  Sym          ⁢                                          ⁢          1          ⁢                      (                                          b                ⁡                                  [                                      g                    ⁡                                          (                      k                      )                                                        ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    1                                    ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    50                                    ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    51                                    ]                                                      )                                              (        1        )                                          d          ⁡                      (                          k              +              50                        )                          =                  Sym          ⁢                                          ⁢          2          ⁢                      (                                          b                ⁡                                  [                                      g                    ⁡                                          (                      k                      )                                                        ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    1                                    ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    50                                    ]                                            ,                              b                ⁡                                  [                                                            g                      ⁡                                              (                        k                        )                                                              +                    51                                    ]                                                      )                                              (        2        )                                          g          ⁡                      (            k            )                          =                  {                                                                      2                  ⁢                  k                                                                              k                  ∈                                      [                                          0                      ,                      24                                        ]                                                                                                                                            2                    ⁢                    k                                    +                  50                                                                              k                  ∈                                      [                                          25                      ,                      49                                        ]                                                                                                          (        3        )            
Expression (1) is used to generate a reference symbol that is adapted for the first-half 50 subcarriers in the event the same information is separated for two carriers and transmitted thereover by the DCM scheme. Expression (2) is used to generate a reference symbol that is adapted for the second-half 50 subcarriers. In the expressions, b[x](0≦x<200) represents 200 information bits, and g(k) represents an index for selecting four bits from 200 bits. Sym1(a, b, c, d) and Sym2(a, b, c, d) are functions for generating a complex-number representing symbol from the same four bits a to d. Respective 16-QAM mappings in the first-half 50 subcarriers and second-half 50 subcarriers have constellation characteristics as shown in FIGS. 6A and 6B. In the event the same information is separated for two carriers and transmitted thereover by the DCM scheme, different 16-QAM mapping processes are performed, such that information of the same four bits are mapped into different transmission symbols (although different 16-QAM mapping processes, the information to be transmitted (or, “transmission information,” hereafter) is the same).
The OFDM symbol contains 100 data subcarriers (described above). In DCM, 100 symbols d(0) to d(99) are OFDM modulated and then are transmitted. FIG. 7 shows a DCM subcarrier array.
In the event that, as described above, the transmitter performs mapping of the multibit binary signal into the signal constellation and thereby performs data transmission thereof, the receiver side has to perform demodulation for returning the received signal from the signal point to the original multibit binary signal.
As a demodulation scheme corresponding to the QAM scheme, a scheme called “LLR-used demodulation scheme” (LLR: log-likelihood ratio) is known (see, J. G. Proakis, “Digital Communications (Fourth Edition),” McGraw-Hill, 2001), for example. The LLR-used demodulation scheme calculates distances between received symbols and respective reference symbols to obtain most closest signal points, and performs demapping thereof in accordance with the calculation results. More specifically, in the LLR-used demodulation scheme, calculations are necessary to search for a signal point closest to a received symbol in the signal constellation. For example, in 16 QAM in which transmission bits b0, b1, b2, and b3 are mapped into 16 points in the signal constellation, an expression for demapping the transmission bit b0 is represented as shown below (Expression (4)).
                                                        LLR              =                              log                ⁡                                  (                                                            P                      ⁡                                              (                                                                              y                            ⁢                                                                                                                  ⁢                            1                                                    ,                                                                                    y                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                              ❘                                                            ⁢                              b                              ⁢                                                                                                                          ⁢                              0                                                        =                            1                                                                          )                                                                                    P                      ⁡                                              (                                                                              y                            ⁢                                                                                                                  ⁢                            1                                                    ,                                                                                    y                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                              ❘                                                            ⁢                              b                              ⁢                                                                                                                          ⁢                              0                                                        =                            0                                                                          )                                                                              )                                                                                                        =                                                log                  ⁡                                      (                                          P                      ⁡                                              (                                                                              y                            ⁢                                                                                                                  ⁢                            1                                                    ,                                                                                    y                              ⁢                                                                                                                          ⁢                              2                              ⁢                                                              ❘                                                            ⁢                              b                              ⁢                                                                                                                          ⁢                              0                                                        =                            1                                                                          )                                                              )                                                  -                                  log                  ⁡                                      (                                          P                      ⁡                                              (                                                                              y                            ⁢                                                                                                                  ⁢                            1                                                    ,                                                                                                                    y                                ⁢                                                                                                                                  ⁢                                2                                                            ❘                                                              b                                ⁢                                                                                                                                  ⁢                                0                                                                                      =                            0                                                                          )                                                              )                                                                                                          (        4        )            In the above:
                              P          ⁡                      (                                          y                ⁢                                                                  ⁢                1                            ,                                                y                  ⁢                                                                          ⁢                  2                  ⁢                                      ❘                                    ⁢                  b                  ⁢                                                                          ⁢                  0                                =                1                                      )                          =                              ∑                                          b                ⁢                                                                  ⁢                0                            =              1                                ⁢                                    1                                                2                  ⁢                  π                  ⁢                                                                          ⁢                                      σ                    2                                                                        ⁢                          exp              ⁡                              (                                  -                                                                                                                                                                                                  y                              ⁢                                                                                                                          ⁢                              1                                                        -                                                          a                              ⁢                                                                                                                          ⁢                                                              1                                ·                                s                                                            ⁢                                                                                                                          ⁢                              1                                                                                                                                2                                            +                                                                                                                                                            y                              ⁢                                                                                                                          ⁢                              2                                                        -                                                          a                              ⁢                                                                                                                          ⁢                                                              2                                ·                                s                                                            ⁢                                                                                                                          ⁢                              2                                                                                                                                2                                                                                    2                      ⁢                                                                                          ⁢                                              σ                        2                                                                                            )                                                                        (        5        )                                          P          ⁡                      (                                          y                ⁢                                                                  ⁢                1                            ,                                                y                  ⁢                                                                          ⁢                  2                  ⁢                                      ❘                                    ⁢                  b                  ⁢                                                                          ⁢                  0                                =                0                                      )                          =                              ∑                                          b                ⁢                                                                  ⁢                0                            =              0                                ⁢                                    1                                                2                  ⁢                  π                  ⁢                                                                          ⁢                                      σ                    2                                                                        ⁢                          exp              ⁡                              (                                  -                                                                                                                                                                                                  y                              ⁢                                                                                                                          ⁢                              1                                                        -                                                          a                              ⁢                                                                                                                          ⁢                                                              1                                ·                                s                                                            ⁢                                                                                                                          ⁢                              1                                                                                                                                2                                            +                                                                                                                                                            y                              ⁢                                                                                                                          ⁢                              2                                                        -                                                          a                              ⁢                                                                                                                          ⁢                                                              2                                ·                                s                                                            ⁢                                                                                                                          ⁢                              2                                                                                                                                2                                                                                    2                      ⁢                                                                                          ⁢                                              σ                        2                                                                                            )                                                                        (        6        )            
In this case, y1 and y2, respectively, are received symbols corresponding to transmission symbols x1 and x2 obtained in the manner that the same transmission information x is separated for two carriers and subjected to two different 16-QAM mapping processes; a1 and a2 are complex impulse responses of propagation channels that have been measured for the respective carriers; and s1 and s2, respectively, are 16-QAM signal points serving as references, and σ2 is a noise power. Demapping of the other transmission bit b1, b3 can be represented by a similar expression, but it is omitted from here.
Expression (5) is an expression for calculating a likelihood that a transmission bit b0 of the received symbol (y1, y2) is 1. More specifically, the likelihood is calculated for the respective signal point (s1, s2) in which b0=1, a distance on a complex plane between the received symbol (y1, y2) and a complex symbol (a1 s1, a2·s2) representing “b0=1” is obtained, and the distances are summed. FIG. 8 shows the relationships between transmission signal constellations and transmission bits. The symbols in which b0=1 in Expression (5) correspond to reference signal points mapped into portions indicated with “b0=1” in two signal constellations y1 and y2 shown in FIG. 8, and correspond to eight symbols in 16 QAM.
Similarly, Expression (6) is an expression for calculating a likelihood that a transmission bit b0 of the received symbol (y1, y2) is 0. More specifically, the likelihood is calculated for the respective signal point (s1, s2) in which b0=0, a distance on a complex plane between the received symbol (y1, y2) and a complex symbol (a1□s1, a2□s2) representing “b0=0” is obtained, and the distances are summed. The symbols in each of which b=0 in Expression (6) correspond to reference signal points mapped into portions indicated with “b0=0” in two signal constellations y1 and y2 shown in FIG. 8, and correspond to eight symbols in 16 QAM.
In the step of the LLR calculation shown in Expression (4), “b0=1” or “b0=0” is selected so that an after-reception posteriori condition probability is maximized. More specifically, when the respective probabilities that b0=1 and b0=0 are calculated for the received symbol (y1, y2) in accordance with Expressions (5) and (6), logarithmic conversions and a subtraction is carried out in Expression (4). Thereby, a soft determination value can be obtained.
Also for the bit value of respective one of the other bit positions b1 to b3, LLR calculations similar to the above are carried out and the soft determination is made, thereby making it possible to demap the received symbols (y1, y2) to highest-probability reference signal points.
FIGS. 9 to 13 each show the state where received 16-QAM symbols are demodulated. In each of the states as shown in the views, the amplitude and the rotation are imparted to 16 grating points corresponding to 16-QAM reference symbols in the transmission signal constellation in accordance with a predicted channel impulse response and phase rotation amount, whereby 16 reference symbols are obtained. Then, a received symbol indicated by reference character r is plotted, and a closest reference symbol is detected, and demapping of transmission bits is carried out.
According to Expression (4), the bit values of the respective transmission bits b0 to b3 are subjected to likelihood determinations in accordance with the results of the calculations of the distances between the received symbol and 16 closest reference symbols, whereby the 16-QAM demodulation is carried out. It is known that Expression (4) is an expression for completely demodulating a received symbol, and optimal characteristics can be obtained according to the LLR-used demodulation scheme.
FIG. 14 is a schematic view of a circuit configuration that executes the demodulation of a received 16-QAM mapped symbol by using the LLR-used demodulation scheme. As can be seen in FIG. 14 as well, in the LLR-used demodulation scheme, a drawback exists in that since calculations of, for example, exp ( ) and a summing circuit are involved, the circuit size has to be increased, in which there arises a technical problem of how to simplify the calculations.
In the MB-ODFM communication scheme under study in conjunction with IEEE 15.3, frequency diversity is used in DCM. In this case, the same information x is separated for two carriers, and distinct 16-QAM mapping processes are carried out for the respective streams of the information (see FIGS. 6A and 6B), and the streams of the information are transmitted as distinct transmission symbols (x1, x2). As such, whereas it is sufficient for ordinary 16 QAM to process one received symbol, it is necessary for MB-OFDM to two received symbols (y1, y2) to demodulate the single bit b0, as shown in Expressions (5) and (6). Thus, with the DCM scheme being employed, since the number of calculations for demodulation is doubled, such that the issue of the circuit size becomes even more serious.
Further, in Expression (5), “exp(−x)” exponent calculations are carried out for the respective eight 16-QAM reference symbols, and the calculation results are summed (see FIG. 14). When the value of x is large, the result of calculation of exp(−x) is very small to a negligible extent. As such, in Expression (5), the calculation of (y1−a1·s1)2 is carried out for the symbol in which b0=1, only the value of (y1−a1·s1)2 to be minimum is used, but other values are set to 0, thereby making it possible to significantly reduce the number of calculations necessary in the LLR-used modulation scheme (see, Japanese Unexamined Patent Application Publication No. 2002-330188, par. 0015). Expression (7) below is an expression used in a scheme of that type to obtain the probability that the transmission b0=1. In Expression (7), Min represents a function for selecting the minimum value. Also the Expression (6) for obtaining the probability of a transmission bit having “b0=0” can be substituted with a similar expression. Further, a similar expression can be used for each of the other transmission bits b1 to b3, but presentation thereof is omitted herefrom.P(y1,y2|b0=1)=Min{|y1−s1·a1|2+|y2−s2·a2|2}  (7)
Nevertheless, however, a problem arises with Expression (7) in that since complex multiplication operations and division operations are necessary, the circuit size has to be increased. FIG. 15 is a schematic view of a circuit configuration for demodulating received 16-QAM mapped symbols by using Expression (7). As can be seen in the drawing figure, squaring devices are necessary corresponding in number to symbols (eight in the case of 16 QAM).